1. Field of the Invention
The present invention relates to a method for curing ultraviolet-curable silicone compositions. More particularly, this invention relates to a method for curing ultraviolet curable silicone rubber compositions that yield, upon exposure to a particular type of UV radiation, cured silicone having a low-tack surface which is harder than the interior.
2. Description of the Related Art
UV-curable silicone compositions that cure rapidly into cured silicone under UV illumination are widely used in industrial applications. These UV-curable silicone compositions are exemplified by UV-curable silicone compositions in which the base ingredient is acrylamide-functional organopolysiloxane (Japanese Patent Publication Number Sho 57-52371 (1982) and European Patent Number EP 0400785) and by UV-curable silicone compositions in which the base ingredient is itself a composition comprising organopolysiloxane having at least 2 mercaptoalkyl groups in each molecule and organopolysiloxane having at least 2 alkenyl groups in each molecule (U.S. Pat. No. 4,707,503). The irradiation of these UV-curable silicone compositions with ultraviolet light is carried out using the well-known light sources, such as high-pressure mercury lamps, medium-pressure mercury lamps, low-pressure mercury lamps, hydrogen-discharge tubes, xenon-discharge tubes, metal halide lamps, and so forth.
However, the direct exposure of UV-curable silicone compositions to UV light from the aforesaid light sources yields cured silicone with a very tacky surface. This can cause handling to be quite difficult. In addition, such surfaces readily pick up dirt and dust, which causes a substantial deterioration in appearance. These problems are particularly acute with lower-hardness cured silicones. For example, when a silicone gel is formed on the surface of an optical semiconductor element using a UV-curable silicone composition, the dust and dirt picked up by the surface can cause a major decline in the performance of the element.
The inventors carried out extensive investigations in order to solve the above-described problem. As a result, it was discovered that UV light in the short wavelength region is a major contributor to curing at the surface of UV-curable silicone compositions. The inventors also found that low surface tack cured silicone whose surface is harder than its interior can be produced by exposure to UV light at a short-wavelength exposure dose/long-wavelength exposure dose ratio within a particular range. The invention was achieved based on these discoveries.
In specific terms, then, the present invention takes as its object the introduction of a method for curing an ultraviolet-curable silicone compositions that characteristically produces a cured silicone having a low-tack surface which is harder than the interior. This cured silicone is produced by exposing a UV-curable silicone composition to a particular type of UV radiation.